Biology · MU Sofia

24. Factors influencing allele frequencies: mutations, migrations, natural selection.

Explains how allele frequencies shift over time through mutation, migration (gene flow) and natural selection, in that order. Mutation covers the mutation rate, its usual neutral or harmful effects and why small populations are more vulnerable. Gene flow covers its effect on diversity and similarity between populations. Natural selection covers relative fitness, the selection coefficient, dominant versus recessive alleles, and heterozygote advantage with sickle-cell and Duffy examples. Four mnemonics summarise the points.

Inside: 3 sections · 1 labelled picture · 12 flashcards · 8 min

Topic 25 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 3 sections, about 1,139 words (8 min of reading), with 12 flashcards.

In this note

  1. Factors Influencing Allele Frequencies: Mutations, Migrations, Natural selection
  2. Mutations
  3. MNEMONICS

Factors Influencing Allele Frequencies: Mutations, Migrations, Natural selection

Allele frequencies within a population are not fixed. They can change over time due to several evolutionary forces. These forces disrupt the assumptions of the Hardy-Weinberg equilibrium, which predicts that allele frequencies will remain constant in the absence of evolutionary influences. When these forces act on a population, they initiate evolutionary change by altering allele distributions across generations.

Among the primary evolutionary forces, this summary focuses on three major ones: mutations, migration, and natural selection.

Mutations

  • Mutations are changes in DNA that can result in one allele transforming into a different allele.
  • They are the source of all new genetic variation and are essential for evolution to occur.
  • Without mutations, there would be no new alleles for natural selection or other evolutionary forces to act upon.
  • Although mutation events are relatively rare, they occur in every generation across all organisms.
  • Most mutations are either neutral or harmful, but occasionally a beneficial mutation may arise and persist if favored by natural selection.
  • Mutations do not usually have a large effect on the gene pool unless they offer a selective advantage.
  • The mutation rate is the probability that a normal allele becomes a mutant allele. This rate is measured per gamete or cell division per generation and is represented by the symbol "mu."
  • Mutation rates vary between organisms and even between genes within a single organism. For instance, longer genes are more prone to mutations.
  • Organisms with less complex DNA repair and replication systems tend to have higher mutation rates. For example, mutation rates are significantly lower in Paramecium than in humans.
  • Environmental and genetic factors that disrupt DNA replication or repair can increase mutation rates.
  • While some mutations drive evolutionary adaptation, an accumulation of too many mutations can lead to genetic disorders or birth defects.
  • Small populations tend to be more affected by mutations because they have fewer individuals to buffer the effects of harmful changes.
  • Initially, scientists believed mutation rates were stable and could be used like clocks to date species divergence. However, newer research has shown that these rates can vary significantly between species.
  • When a mutation is beneficial, it can gradually replace the original allele in the population.
  • When a mutation is harmful, it can still persist at low levels in the population if it is continuously introduced by mutation despite being selected against.

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